The present invention generally relates to radio communications and in particular to improved communication with scanning antennas.
Conventional communication systems for Cellular and Personal Communication Systems (PCSs) use a series of communications networks to allow users to communicate with one another. These networks include a number of Mobile Switching Centers (MSCs) that connect users to Private Switched Telephone Networks (PSTNs). In addition, the MSCs are connected to a number of base stations. The base stations are located in the various cells of the network in order to provide network coverage in the area that is local to the base station. The base stations are typically equipped with antennas that allow communication between the base stations and mobile users within the cell where the base station is located. The base stations in turn communicate with the MSCs and other base stations to allow PCS users to communicate with other PCS and PSTN users.
Conventionally, dual polarized phased array antennas are used to transmit and receive RF communications at the base station. These antennas are commonly located on the top of towers and service communication within a cell or micro cell. A phased array is an antenna having two or more driven elements directly connected to a feed line which is in turn connected to a feed network. Conventionally a plurality of driven elements are used for antennas adapted for use in cellular communications at towers connected with the base stations. The driven elements are fed with a particular relative phase and are spaced at a predetermined distance from each other. This arrangement results in a directivity pattern exhibiting gain in some directions and little or no radiation in others.
In order to provide polarization diversity, orthogonal polarization is commonly used to provide non-correlated paths. The direction of polarization is commonly measured from a fixed axis and can vary as required by system specifications. The polarization direction can extend from vertical polarization (e.g., zero degrees) to horizontal polarization (e.g., 90 degrees). Most conventional systems use slant polarization of .+-.45 degrees to -45 degrees in order to isolate communications between one of two communication ports. If the antenna receives or transmits signals of two polarizations that are normally orthogonal, they are referred to as dual polarized antennas. Dual polarized antennas are required to meet specified port-to-port coupling or isolation requirements between dual ports that are connected to the feeder network. Conventionally, port-to-port isolation is required to be -30 db.
It is therefore desirable to have very low port isolation. One method of improving port-to-port isolation of dual polarized antennas is to fix parasitic elements in phased array fixed beam antennas. The parasitic element is an electrical conductor or circuit that is not directly connected the feed line (or communications ports) of the antenna. The parasitic element is used to perturb the electromagnetic field in such a way that port isolation i s increased. This is not to be confused with parasitic elements used in Yagi antennas that are used to provide directivity and power gain and operate by EM coupling to the driven antenna elements. For example, these parasitic elements placed parallel to the driven elements, at a predetermined distance and having a predetermined length, but not connected to anything, cause a radiation pattern to show gain in one direction and loss in the opposite direction. When a gain is produced in the direction of the parasitic element, the parasitic element is a director. When the gain is produced in the direction opposite of the parasitic element, the parasitic element is known as a reflector and provides a canceling signal.
In marked contrast, parasitic elements as used in the present invention, have been used to improve port-to-port isolation in dual polarized fixed beam antennas. The parasitic element is carefully placed on the antenna at a spot that is empirically determined to reduce the isolation between ports of the feed network to the antenna. The parasitic element is then fixed in place at the position that is determined to provide the best port-to-port isolation.
Although it is desirable to improve port-to-port isolation, many cellular/PCS communication systems use a scanning antenna array arrangement of dual polarized antennas. Scanning antenna arrays may be adjusted by repositioning the arrays to avoid channel interference with other broadcast stations and their associated antennas caused by overcrowding and to optimize coverage within a specific area serviced by the antenna. An example of a scanning antenna is a down tilt antenna. Down tilt antennas help reduce the problem of cell site overlap by adjusting the vertical scan angle to carefully position the antenna in order to provide the necessary coverage while avoiding interference with other microcells within the network and adjacent competing networks.
Conventionally, while fixed parasitic elements are used to establish low port-to-port isolation for dual polarized antennas in fixed beam antennas, the improvement is not evident in scanning beam antennas, such as downtilt antennas because the improved isolation is not uniform over the full scan range of the downtilt, for example. In fact, the isolation is actually degraded for certain angles by destructively adding to the isolation response or by changing the mutual coupling between ports in such a way that reduces the quality of the overall isolation response. As the isolation response changes as a function of the tilt angle and therefore conventional mechanisms providing a fixed canceling response will not work effectively to reduce the isolation response over a varying scan angle. Therefore, parasitic elements are not used to improve isolation response in scanning antennas.